The present invention relates to a process and an apparatus for the continuous chemical conversion of materials, in particular to a process and an apparatus for the conversion of waste cellulose to glucose by acid hydrolyzation and more particularly to an improvement in the process and apparatus disclosed in copending U.S. application Ser. No. 131,340 filed on the same day as this application.
Acid hydrolysis of cellulose has been extensively studied for the better part of the century, particularly in connection with the manufacturing from ethanol from wood wastes. It has long been known that cellulose can be hydrolyzed in acid solutions and converted to its monomer, glucose, and the reaction has been experimentally investigated since this discovery. The reaction results from the fact that the monomers of cellulose are anhydroglucose units, and that during hydrolyzation, a water molecule is added to the cellulose monomer unit to obtain the heavier molecular weight glucose.
Recently, there has been a growing interest in the utilization of waste cellulose for energy production, because of the possibility of producing ethyl alcohol from glucose, and for the purposes of materials recovery.
While the acid hydrolysis of cellulose is heterogeneous, it can be regarded as a homogeneous reaction, provided that the cellulose reactant is dispersed in the form of fine particles, i.e., 200-mesh or less. The kinetically predicted sugar yields assume that the cellulose reactant has appropriate chemical reactivity for the acid hydrolysis. The technical problems of cellulose hydrolysis are to a great extend due to the fact that this is not the case. The lack of an adequate amount of chemical reactivity in cellulose is called lack of accessibility. This is related to the highly inert character and crystalline organization on a molecular level of the high molecular weight cellulose, and also the presence of lignin. Hydrogen-bonding almost certainly plays a very important role in the structure of cellulose, and may be a key factor in explaining its chemical inertness.
In general, mechanical treatments, such as, for example, intensive ball milling to sizes below 60 mesh, have been found to be technically effective, but at a high cost which renders any process economically prohibitive. Treatment with high-energy ionizing radiation on the order of 100 megarads has been shown to be effective, however the cost of such large doses of ionizing radiation is too high for industrial usage.
While heretofore successful batch-wise production of glucose from cellulose has been carried out by the acid hydrolysis of waste cellulose, this type of process and the apparatus for carrying it out are insufficient for commercial production.
The aforementioned copending U.S. application Ser. No. 131,340 teaches a process and apparatus for the quasi-continuous conversion of fibrous material to a derivative thereof and in particular for the quasi-continuous acid hydrolysis of cellulose to glucose, based upon the use of a hydrolysis reactor which is capable of feeding, conveying and discharging hydrolysable cellulosic materials continuously while maintaining appropriate temperatures and/or pressures in the reaction zone thereof.
According to that disclosure the hydrolysis reactor is a Werner and Pfleiderer ZDS-K 53 (53 mm) corotational two screw extruder which was selected because of its capacity for conveying, mixing and extruding the required amounts of cellulosic feedstock. The extruder allows accurate control of temperature, pressure, residence time, etc. The extruder has the working elements of intermeshing twin screws which eliminate material buildup in the processing section and make feasible close control of resisdence time, etc., with intensive mixing.
For the quasi-continuous processing of materials, the reactor was coupled with an appropriate feeding mechanism for cellulose slurries and a discharge system for reacted material while maintaining the pressure and/or temperature in the reaction zone. In particular, the feeding means included a steam jacketed crammer feeder also produced by the Werner & Pfleiderer Corp.
In order to maintain the pressure in the reaction zone during the process, pressure is maintained at the inlet to prevent egress of the material through the crammer feeder by a dynamic seal in the form of a densified plug of material within the inlet zone of the reactor. Simultaneously, quasi continuous discharge of the hydrolyzed material was accomplished while maintaining the pressure by the use of a discharge system comprising a hydraulically powered activator and a ball valve, in particular the Kamyr Intensive Service 2" ball valve.
The dynamic seal was achieved by the formation of a dynamic plug zone in the extruder, at the inlet end of the reaction zone. The dynamic seal was formed in the conventional manner, by utilizing a left handed screw thread in the dynamic seal zone with right handed threads disposed downstream and upstream thereof.